A thermal printhead generally comprises a row of closely spaced resistive heat generating elements which are selectively energized to record data in hard copy form. The data may comprise text, bar code or pictorial information. In operation, the thermal printhead heating elements selectively receive energy from a power supply through control circuits in response to the stored data information. The heat from each energized element may be applied directly to thermal sensitive material. Alternatively, the heat may be applied to a dye coated web to transfer the dye to paper or other receiver material.
In dye transfer printers, the density of the printed dye is a function of the temperature of the heat element and the time the dye coated web or carrier is heated. The quantity of dye transferred to an image pixel of the receiver is directly related to the amount of heat supplied to the carrier. By varying the heat applied by each heating element to the carrier, a variable dye density image pixel is formed in the receiver. In this way, a continuous tone image may be produced. Such continuous tone apparatus can be used to print both monochrome and color images. For monochrome images, a single image with a black dye is printed. For color images, a cyan, magenta and yellow or a blue, green and red system may be used. In one multiple pass arrangement, a cyan image is printed first. Magenta and yellow images are then superimposed on the cyan image to form the color print. Single pass systems may also be used.
Multiple gray scale images produced by thermal printers generally do not provide optimum sharpness and may include unwanted artifacts. Image signal alteration is usually needed to obtain satisfactory printed images. Some thermal image printers that support multiple gray levels contain a simple edge enhancement capability. By using a few neighboring pixels on a single line to perform enhancement, processing and memory requirements are kept to a minimum. These printers may also perform corrections for differences in the optical density produced by the head elements. U.S. Pat. No. 4,774,528 (issued to Nobuhisa Kato on Sep. 27, 1988), for example, discloses thermal recording apparatus in which the black density of pixels to be recorded by thermal recording elements are compared to reference density levels. A value representing the number of pixels having density levels in certain ranges as a result of the comparison is accumulated. The density value range is used to adjust the pulse width of energizing pulses. In this way, compensation is provided for voltage fluctuations at the printhead heat elements due to the number of recording elements energized at one time.
U.S. Pat. No. 4,746,931 (issued to Akira Okuda on May 24, 1988) discloses a thermal head temperature control device having a programmed processor with fixed data stored for bar code and ordinary character printing. The fixed data is selected as a function of the print type to control the power supplied to the heat elements of a thermal printhead. There is no arrangement, however, for altering image signals in response to the values of individual pixel image signals to control printing.
U.S. Pat. No. 4,786,917 (issued to Edward A. Hauschild on Nov. 22, 1988) discloses a signal processing arrangement for a thermal printer which incorporates a programmed processor. The processor sets up lookup tables to correct contrast and color of image signals to be applied to a thermal printhead. The image signals are then modified in response to the difference between corresponding image signals for the present print line and the immediately preceding print line to provide edge enhancement.
The aforementioned patents describe techniques that improve thermal printing by modifying the image signals of the print line being applied to the thermal printhead. The modifications are made in response to the type of pixel image signals of the print line currently being applied to the thermal printhead or in response to the values of the pixel image signals of the current print line and immediately preceding print line. It is well known, however, that each pixel of an image is related to surrounding pixels of more remote print lines both preceding and succeeding the current print line. Consequently, improvement in the print image based on pixels of immediately successive pairs of print lines is limited. It is desirable, therefore, to further improve thermal printer images by modifying each pixel image signal in response to the values of the surrounding pixel image signals in a plurality of preceding and succeeding print lines.